Style.min.css 13.satır
.header-19 .menu__container .menu-item-title:after {
content: "";
width: 0;
height: 1px;
bottom: 0;
position: absolute;
left: auto;
right: 0;
z-index: -1;
-webkit-transition: width .6s cubic-bezier(.25,.8,.25,1) 0s;
transition: width .6s cubic-bezier(.25,.8,.25,1) 0s;
background: currentColor;
}


Bu kodu bulup aşağıdaki kodu en altına ekleyin.

background: rgb(109,255,113);
background: linear-gradient(90deg, rgba(109,255,113,1) 0%, rgba(255,75,75,1) 33%, rgba(0,155,255,1) 66%, rgba(106,0,255,1) 100%);